1. Field of the Invention
This invention relates to the purification of caustic liquors and high caustic streams, especially those relating to the production of aluminum hydroxides (also called alumina trihydrates) and aluminas from bauxite according to the Bayer process (sometimes referred to as Bayer liquor streams). The invention further relates to means for making an intermediate grade aluminum hydroxide/hydrate product in terms of brightness, i.e., having a whiteness level of about 85% or higher, more preferably about 90 or 92% whiteness, based on a 100% TiO.sub.2 standard and a target adsorbence level of about 0.20. For purposes of this invention, the terms "brightness" and "whiteness" are used interchangably.
2. Technology Review
The recovery of aluminum hydroxide from bauxite according to the Bayer process is achieved by digesting hydroxide-containing ores with a caustic liquor. A major portion of alumina dissolves in this liquor while most unwanted ore constituents, sometimes called "red mud", remain undissolved. After bauxite is pressure digested with a caustic such as sodium hydroxide, red mud may be removed from this liquor. Aluminum hydroxide is then separated from a liquor of supersaturated sodium aluminate, also known as "green" or "pregnant" liquor, typically by precipitation. During such precipitation, the supersaturated sodium aluminate is cooled and mixed with a slurry of aluminum hydroxide particles acting as seed material, or seed stock, to induce the formation of more aluminum hydroxide. After precipitation, the slurry is pumped through a classifier system where a coarse fraction of crystallized aluminum hydroxide is separated from the liquor. The fine fractions of crystallized (or precipitated) aluminum hydroxide are further classified into two finer fractions called "secondary seed" and "tray seed", the latter being the finest fraction from this classifier system. These secondary and tray seeds are often recycled to a precipitator to act as seed for further aluminum hydroxide precipitation. The resulting spent sodium aluminate may be recycled to a digester for mixing with new (or incoming) bauxite.
Sodium aluminate liquors can also be made by digesting precipitated aluminum hydroxide (primary), secondary seed, tray seed or combinations thereof in a method known as redigestion. Some Bayer plants produce excess seed (generally tray seed) which they may thereafter redigest by recycling to bauxite digesters or through a separate, specially designed digester.
High levels of impurities are undesirable in the sodium aluminate liquor used to make aluminum hydroxide because such impurities decrease the whiteness or color purity of the hydroxide/hydrate precipitated therefrom. It is desirable, therefore, to minimize the presence of such impurities in green sodium aluminate liquors before crystallization takes place. Such impurities typically cause the aluminum hydroxide produced to have a whiteness level of about 75% or less based on a 100% TiO.sub.2 standard thus prohibiting their use in many applications where generally higher brightness levels (about 80% or above) are required. The present invention produces aluminum hydroxide having whiteness levels of about 85% or more based on the same 100% TiO.sub.2 reference standard. For some embodiments, whiteness levels of about 90 or 92% are achieved consistently.
When organic and inorganic impurities are present, lower liquor productivity and reduced alumina purity result. Organic impurities may cause such other complications as: lower alumina yields; excessively fine hydroxide particles; the production of colored liquors and aluminum hydroxide; lower red mud settling rates; caustic losses due to sodium organic formation; an increase in liquor density; increased viscosities; higher boiling points; and unwanted liquor foaming.
Numerous methods are known for removing colorants from a sodium aluminate liquor. These include Australian Patent No. 12085/83 which teaches treating liquors with reactive MgO or Mg(OH).sub.2 before calcining at 900.degree. C. or more. Schepers et al. U.S. Pat. No. 4,046,855 also discloses treating aluminate liquors with a magnesium compound to remove organic colorants therefrom. Japanese Patent No. 57-31527 produces an aluminum hydroxide of high purity by adding one or more alkaline earth metal compounds to such solutions. Representative additives include oxides, hydroxides, carbonates, silicates and nitrates of magnesium or calcium; and barium carbonates, silicates, nitrates or sulfates.
German Patent No. 3,501,350 discloses adding a mixture of calcium oxide/hydroxide and kierserite (MgSO.sub.4.H.sub.2 O) to Bayer liquors for lowering impurity levels, especially iron contents. In Goheen et al. U.S. Pat. No. 4,915,930, an aluminum hydroxide of improved whiteness is produced by contacting liquor streams with a mixture of tricalcium aluminate and unactivated hydrotalcite. Finally, in Nigro U.S. Pat. No. 5,068,095, caustic solutions, are treated with calcined hydrotalcite to remove colorants, especially iron. High dosages of about 10 g/l were preferred to make hydroxides/hydrates with about 80% whiteness levels or higher. The present invention represents an improvement over the aforementioned Nigro et al. method.